N-ary Tree Preorder Traversal LeetCode solution in Java

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class Solution {
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  public List<Integer> preorder(Node root) {
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    if (root == null) return new ArrayList<Integer>();
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    Stack<Node> stk = new Stack<Node>();
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    ArrayList<Integer> arr = new ArrayList<Integer>();
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    stk.push(root);
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    Node ref;
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    while (!stk.empty()) {
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      ref = stk.pop();
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      // System.out.println(ref.val);
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      arr.add(ref.val);
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      for (int i = ref.children.size() - 1; i >= 0; i--) {
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        stk.push(ref.children.get(i));
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      }
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    }
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    return arr;
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  }
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}